There is no evidence that weather or a mechanical discrepancy contributed to the accident. The analysis will therefore focus on the pilot's applied IFR/night take-off technique, the role and influence of the quasi co-pilot, the communications between Airco Aircraft Charters and Northern Sky Aviation, the Airco dispatcher's request to change the flight plan, and the effect of the overweight condition of the aircraft on the departure. Individually these factors would likely not have been significant enough to cause an accident. When combined with dark ambient conditions and an uphill take-off toward rising terrain, these factors collectively established a window of opportunity for an accident to occur. The pilot's night departure technique is considered to be the active failure in this accident. Night departures in dark conditions require full use of the aircraft flight instruments, and it is essential that the pilot achieve and maintain a positive rate of climb at a safe climb speed after lift off. In the absence of outside visual cues, the pilot must rely on the aircraft instruments to maintain airspeed and attitude and to overcome any false sensations of a climb. The pilot was either relying on outside visual cues during the initial climb and/or using only a partial instrument panel scan while being influenced by the false-climb illusion. Pilots can overcome false sensations by flying the aircraft with reference to the altimeter, VSI, attitude indicator, and airspeed indicator, which, in this case, would likely have allowed the pilot to detect that the aircraft was not established in a climb. The appropriate technique would have been to climb at the aircraft's best rate or best angle of climb speed until above all obstacles rather than become preoccupied with reaching blue line. The pilot may have intended to reach the blue line speed of 107 KIAS soon after take-off because he was aware that the aircraft was at or above the approved take-off weight, and he did not want to climb at a minimum climb airspeed. The role of the Northern Sky Aviation company representative/quasi co-pilot is somewhat ambiguous, and his presence does not appear to have contributed significantly to the safety of the flight. Because he was not familiar to the captain and because he was not delegated flight crew responsibilities, his participation during the flight created a situation of crew resource mismanagement. The co-pilot's remarks regarding the weight and flight plan changes at High Level appear to have encouraged the captain to cancel the planned fuel stop in Peace River. In discussing the proposed changes the co-pilot did not advise the captain that if weight was a concern he could remain in Rainbow Lake and thereby reduce the take-off weight by approximately 170 pounds. The co-pilot's apparent well-intentioned advice on the frost heave near the threshold of runway 09 influenced the captain's decision to take-off on runway 27, which was uphill toward rising terrain and with no lights visible after departure. The co-pilot's attention to the flight instruments and aircraft climb profile during the initial climb phase of flight was such that he did not have time to intervene with the appropriate actions to prevent the aircraft from striking the trees. The aircraft had been modified to permit take-off at a weight of 368 pounds higher than that approved by the original type certificate, without an increase in engine power. It is estimated that the actual weight of the aircraft at take-off was approximately 315 pounds above the prescribed increased gross weight, and that the C of G was at or beyond the rear limit. This would have increased the take-off distance and reduced the climb performance of the aircraft. The extent to which the aircraft performance was degraded was not determined. The Airco dispatcher's telephone request to the captain in High Level to add sufficient fuel in order for the flight to not stop at Peace River contributed to the aircraft being overweight on the departure from Rainbow Lake. Communication between Northern Sky and Airco Aircraft Charters with regard to the duties of the co-pilot and the weight of the aircraft at departure from Rainbow Lake were inadequate. Both companies were familiar with Piper PA-31-350 capabilities, and the weight and balance calculations performed prior to the aircraft leaving Edmonton indicated that the trip would have to be accomplished VFR, with a fuel stop at Peace River, to accommodate the passenger load. Critical information, such as the option of dropping the Northern Sky co-pilot in the event of an overweight aircraft condition, was never provided to Airco. The Airco pilot, who was the final decision-maker, was put in the position of having to balance the conflicting objectives of operating the aircraft within the prescribed weight limits and satisfying the customer demands. He was relatively inexperienced on Piper PA-31-350 aircraft, having flown less than 100 hours on the type, and because he had not previously flown a Northern Sky trip he was unfamiliar with the routine of the Northern Sky daily flights. He was aware that Northern Sky often chartered an Airco aircraft for the daily flight, and may therefore have felt peer pressure to fly the trip as he perceived other pilots had in the past.Analysis There is no evidence that weather or a mechanical discrepancy contributed to the accident. The analysis will therefore focus on the pilot's applied IFR/night take-off technique, the role and influence of the quasi co-pilot, the communications between Airco Aircraft Charters and Northern Sky Aviation, the Airco dispatcher's request to change the flight plan, and the effect of the overweight condition of the aircraft on the departure. Individually these factors would likely not have been significant enough to cause an accident. When combined with dark ambient conditions and an uphill take-off toward rising terrain, these factors collectively established a window of opportunity for an accident to occur. The pilot's night departure technique is considered to be the active failure in this accident. Night departures in dark conditions require full use of the aircraft flight instruments, and it is essential that the pilot achieve and maintain a positive rate of climb at a safe climb speed after lift off. In the absence of outside visual cues, the pilot must rely on the aircraft instruments to maintain airspeed and attitude and to overcome any false sensations of a climb. The pilot was either relying on outside visual cues during the initial climb and/or using only a partial instrument panel scan while being influenced by the false-climb illusion. Pilots can overcome false sensations by flying the aircraft with reference to the altimeter, VSI, attitude indicator, and airspeed indicator, which, in this case, would likely have allowed the pilot to detect that the aircraft was not established in a climb. The appropriate technique would have been to climb at the aircraft's best rate or best angle of climb speed until above all obstacles rather than become preoccupied with reaching blue line. The pilot may have intended to reach the blue line speed of 107 KIAS soon after take-off because he was aware that the aircraft was at or above the approved take-off weight, and he did not want to climb at a minimum climb airspeed. The role of the Northern Sky Aviation company representative/quasi co-pilot is somewhat ambiguous, and his presence does not appear to have contributed significantly to the safety of the flight. Because he was not familiar to the captain and because he was not delegated flight crew responsibilities, his participation during the flight created a situation of crew resource mismanagement. The co-pilot's remarks regarding the weight and flight plan changes at High Level appear to have encouraged the captain to cancel the planned fuel stop in Peace River. In discussing the proposed changes the co-pilot did not advise the captain that if weight was a concern he could remain in Rainbow Lake and thereby reduce the take-off weight by approximately 170 pounds. The co-pilot's apparent well-intentioned advice on the frost heave near the threshold of runway 09 influenced the captain's decision to take-off on runway 27, which was uphill toward rising terrain and with no lights visible after departure. The co-pilot's attention to the flight instruments and aircraft climb profile during the initial climb phase of flight was such that he did not have time to intervene with the appropriate actions to prevent the aircraft from striking the trees. The aircraft had been modified to permit take-off at a weight of 368 pounds higher than that approved by the original type certificate, without an increase in engine power. It is estimated that the actual weight of the aircraft at take-off was approximately 315 pounds above the prescribed increased gross weight, and that the C of G was at or beyond the rear limit. This would have increased the take-off distance and reduced the climb performance of the aircraft. The extent to which the aircraft performance was degraded was not determined. The Airco dispatcher's telephone request to the captain in High Level to add sufficient fuel in order for the flight to not stop at Peace River contributed to the aircraft being overweight on the departure from Rainbow Lake. Communication between Northern Sky and Airco Aircraft Charters with regard to the duties of the co-pilot and the weight of the aircraft at departure from Rainbow Lake were inadequate. Both companies were familiar with Piper PA-31-350 capabilities, and the weight and balance calculations performed prior to the aircraft leaving Edmonton indicated that the trip would have to be accomplished VFR, with a fuel stop at Peace River, to accommodate the passenger load. Critical information, such as the option of dropping the Northern Sky co-pilot in the event of an overweight aircraft condition, was never provided to Airco. The Airco pilot, who was the final decision-maker, was put in the position of having to balance the conflicting objectives of operating the aircraft within the prescribed weight limits and satisfying the customer demands. He was relatively inexperienced on Piper PA-31-350 aircraft, having flown less than 100 hours on the type, and because he had not previously flown a Northern Sky trip he was unfamiliar with the routine of the Northern Sky daily flights. He was aware that Northern Sky often chartered an Airco aircraft for the daily flight, and may therefore have felt peer pressure to fly the trip as he perceived other pilots had in the past. The pilot was qualified for the flight, and there was no evidence that his performance was degraded by physiological factors. The pilot's take-off technique was not appropriate for a night departure in that he concentrated on reaching blue line speed rather than maintaining a positive rate of climb after take-off. The role of the Northern Sky company representative/co-pilot was ambiguous, and his presence does not appear to have contributed to the safety of the flight. To the extent that the aircraft was examined, there was no evidence that an airframe, engine, or system failure contributed to the occurrence. Airco Aircraft Charters had not been advised by Northern Sky Aviation of the option to drop the Northern Sky company representative/co-pilot in the event that the aircraft was overweight. Using the reported passenger and baggage weights, it is estimated that the aircraft exceeded the maximum take-off weight by approximately 315 pounds and the centre of gravity was beyond the aft limit. An airport inspection completed on 07 April 1998 determined that trees and brush on the approach path to runway 09 violated the approach slope of 2.5% (1:40). The availability and use of a cell phone greatly expedited the rescue. There was no survival gear available for the occupants.Findings The pilot was qualified for the flight, and there was no evidence that his performance was degraded by physiological factors. The pilot's take-off technique was not appropriate for a night departure in that he concentrated on reaching blue line speed rather than maintaining a positive rate of climb after take-off. The role of the Northern Sky company representative/co-pilot was ambiguous, and his presence does not appear to have contributed to the safety of the flight. To the extent that the aircraft was examined, there was no evidence that an airframe, engine, or system failure contributed to the occurrence. Airco Aircraft Charters had not been advised by Northern Sky Aviation of the option to drop the Northern Sky company representative/co-pilot in the event that the aircraft was overweight. Using the reported passenger and baggage weights, it is estimated that the aircraft exceeded the maximum take-off weight by approximately 315 pounds and the centre of gravity was beyond the aft limit. An airport inspection completed on 07 April 1998 determined that trees and brush on the approach path to runway 09 violated the approach slope of 2.5% (1:40). The availability and use of a cell phone greatly expedited the rescue. There was no survival gear available for the occupants. The aircraft was inadvertently flown into trees and the ground, in controlled flight and dark ambient conditions, during a night departure because a positive rate of climb was not maintained after take off. Factors contributing to the accident were the pilot's concentrating on blue line speed rather than maintaining a positive rate of climb, the dark ambient conditions, a departure profile into rising terrain, an overweight aircraft, and crew resource mismanagement.Causes and Contributing Factors The aircraft was inadvertently flown into trees and the ground, in controlled flight and dark ambient conditions, during a night departure because a positive rate of climb was not maintained after take off. Factors contributing to the accident were the pilot's concentrating on blue line speed rather than maintaining a positive rate of climb, the dark ambient conditions, a departure profile into rising terrain, an overweight aircraft, and crew resource mismanagement.